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THC potency and how it's measured

Smokin Moose

Fallen Cannabis Warrior
So what is the truth about cannabis strength, how has it changed over the years? To answer that question, we need to understand a little about what cannabis is and how the authorities have treated it over the years.

How is cannabis strength measured?
Definition: Strength is the amount of drug per volume or weight of a sample. By way of illustration alcohol strength is measured in "ABV" - the percent of alcohol per volume, so a 5% beer will contain 5% alcohol, easy.

As the drug the government associates with cannabis intoxication is THC and we're dealing with a solid substance, the strength of cannabis would be expected to be measured in Mg THC per gram of sample, that's where we hit a problem.

Now it's worth mentioning that cannabis is not simply THC, it's a blend of various active substances, the different ratios of which produce a very different effect on the user. However, only THC is usually measured, which produces the first problem when we want to examine changes in the nature of cannabis over time. No measurements of the amounts of the other active chemicals have ever been made by the authorities on anything like a regular basis. This is at least in part explained by the fact that cannabis is illegal and what measurements there have been made have been first and foremost for enforcement, not quality control reasons.

However, at least the strength of cannabis has been measured in terms of THC per gram of sample? Sadly, no it hasn't.

In 2005, UKCIA asked the government's anti drug advertising agency "Talk to Frank" how cannabis strength is measured, we were asked to write to the Home Office:

UKCIA question:
Further to a phone chat today (Thursday 19th May) to one of your people

I've been hearing a lot about the strength of cannabis of late. This strength has been quoted as "percent THC", can you explain what this means?

With alcohol, strength is measured in percent alcohol by volume (%ABV), but clearly the THC in cannabis doesn't amount to 10% of the volume nor of the weight, so what is it a percentage of?

As a follow-up question, cannabis isn't simply THC, the other main component is CBD which is known to modify the effects of THC. I have also
seen this ratio reported as a percentage, so how is the ratio of THC to CBD measured?

Many thanks​

Home Office reply:

Thank you for your email of 19 May which has been passed to me for reply.

The percent THC is the weight for weight of THC in the dry cannabis sample selected for analysis. A fresh cannabis plant contains a lower proportion
of THC as fresh plant material contains a lot of water.

THC is the main active constituent of cannabis. The.proportion of other constituents of cannabis is therefore not of interest in terms of potency.
The THC acid (delta-9-tetrahydrocannabinolic acid) is converted to THC when cannabis is heated (e.g. in a cigarette) so some scientists use gas chromatography (GC) analysis to measure THC. This method. effectively measures the total available THC, as the sample is heated in the injection port of the GC.

Ratios of cannabinoids are sometimes measured when comparing cannabis samples. The ratios are relative responses.obtained by the particular method of analysis and so do not directly relate to actual ratios of the percentage compositions. Absolute proportions of CBD would not normally be measured.

Richard Mullins
Drug Legislation and Enforcement Unit
Home Office​

So what does the above tell us?

"The percent THC is the weight for weight of THC in the dry cannabis sample selected for analysis. A fresh cannabis plant contains a lower proportion of THC as fresh plant material contains a lot of water."

It's unclear what they mean by "dry". Plant material, of the sort you might buy from your dealer - even if dry in the normal sense - is still composed largely of water, so what do they mean by "dry"? It would seem that what they're talking about is a desiccated sample, that is a sample in which all the water has been removed, in effect destroying the biomass material, leaving the oils produced by the plant. We did ask for confirmation on this point, but received no answer.

It's also worth pointing out here that the measurement is actually referred to as "potency", not strength. This careful use of words is typical of the Home Office when it's being economical with the truth. The reason this is important is because the amount of oils the plant produces is not a constant fraction of the overall weight. It will depend on which part of the plant is sampled and how it's grown.

So the measurement they make is a percentage by weight of the oils in the sample, not of the overall weight of the sample.

Is potency directly comparable to strength? It's not clear that it is. In which case, the strength of cannabis over the years has never really been measured. It's also apparent from the reply that no standard system for making the measurement has been employed ("some scientists use gas chromatography")

Based on this type of measuring regime, it's clear that making meaningful conclusions about changes in strength is going to be difficult.

How are samples selected for measurement?
Here we hit perhaps the biggest problem as no statistically valid monitoring of the cannabis on sale has ever happened. What measurements have been taken have been made on samples seized by the police in raids. How representative these samples are of the general situation is unknown, but as a sampling method it wouldn't be considered reliable enough for serious scientific analysis.

So: to recap:

The property measured - "potency" - bears an uncertain relation to "strength"
There has been no standard methodology for making the measurements over the years.
The samples measured are unlikely to be a statistically valid sample.

So what conclusions can be drawn from all this?

The most authoritative study in recent times was conducted by the European Monitoring Centre for Drugs and Drug Addiction in 2004 - .http://www.emcdda.eu.int/index.cfm?fuseaction=public.AttachmentDownload&nNodeID=2950&slanguageISO=EN An overview of Cannabis potency in Europe.

The report makes it clear that the data is somewhat unreliable and it's therefore difficult to draw firm conclusions, but it accepts there has always been strong cannabis which can compare to today's offerings. It makes the point that home grown is going to be fresher and thus stronger (cannabis "goes off" with time as the THC breaks down).

There is undoubtedly some strong cannabis available these days, but there always has been. It's reasonable to suppose that, because it's fresher there will be a higher proportion of stronger samples (= better quality), but overall there is no evidence to support the claims of a massive increase in strength.

It should also be mentioned that commercial supplies of cannabis - herbal and hash - are often highly contaminated these days, a direct result of police action aimed at disrupting the supply. It's therefore obvious that most consumers aren't getting the higher value strong product.

Has there been any other change in Cannabis over the years?
The most obvious change in recent times in the UK has been the move from imported hash to "home grown" herbal. Originally of course, cannabis was grown outside in fields by people who had a long social history of using it, they knew what they were growing. These days most of the cannabis supplied commercially is grown intensively under lights, perhaps with the use of chemicals such as pesticides. The motivation for large scale grow ops of course is turnover and profit. How these plants compare with the original truly organic product is unknown. As we've never monitored the total composition of the product, we have no way of knowing if there is a difference.

It should be pointed out however that the cannabis grown under lights is still cannabis. Despite some claims in the press, it isn't "genetically engineered" or otherwise mutated. However, it's also fair to point out that the strains have been selected to grow well under these conditions.

No-one has a clue what's going on to be blunt. Prohibition has prevented any proper monitoring of the commercial supply and it's produced a potentially significant change in the way the cannabis is grown.

The original supply of naturally grown cannabis has been all but eradicated because of prohibition and prohibition has created the market for the new versions of so-called "skunk".

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